1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device.
Note that in this specification, the term “semiconductor device” refers to general devices that can function by utilizing semiconductor properties, and substrates over which semiconductor elements such as thin film transistors (TFTs) are formed, liquid crystal display devices driven using TFTs, electroluminescence (EL) display devices driven using TFTs, semiconductor circuits, and electronic devices are all defined as semiconductor devices.
2. Description of the Related Art
In recent years, techniques of manufacturing semiconductor devices have been greatly improved: flexible substrates are capable of reducing size and weight of semiconductor devices, and further achieving flexibility and impact resistance of semiconductor devices, and thus, are considered to be used for semiconductor devices.
As a method for manufacturing a flexible semiconductor device, a technique in which a semiconductor element such as a thin film transistor is formed over a base substrate such as a glass substrate or a quartz substrate, and then the semiconductor element is transferred from the base substrate to another base material (for example, a flexible base material) has been developed. In order to transfer the semiconductor element to another base material, a step for separating the semiconductor element from the base material that is used in forming the semiconductor element is necessary.
For example, Patent Document 1 discloses a separation technique using laser ablation, which is described below. A separation layer formed of amorphous silicon or the like is formed over a substrate, a layer to be separated which is formed of a thin film element is formed over the separation layer, and the layer to be separated is bonded to an object to which the layer is transferred, by an adhesive layer. The separation layer is ablated by laser light irradiation, so that separation is generated at the separation layer.
In addition, Patent Document 2 discloses a technique in which separation is conducted by physical force such as human hands. In Patent Document 2, a metal layer is formed between a substrate and an oxide layer and separation is generated at the interface between the oxide layer and the metal layer by utilizing weak bonding at the interface between the oxide layer and the metal layer, so that a layer to be separated and the substrate are separated from each other.
In the case where separation is conducted by physical force such as human hands, a layer to be separated needs to be curved in order to separate the layer from a base material along a separation layer. The layer to be separated formed in contact with the separation layer is a very fragile thin film with a thickness of about 10 μm in which semiconductor elements including a thin film transistor (TFT), a wiring, an interlayer film, and the like are formed. When the bending stress is applied to the semiconductor elements, breaking or a crack easily occurs in the layer to be separated, which causes malfunctions such as breaking of the semiconductor devices.
A crack caused in a separation step tends to occur from the periphery of a substrate. When the crack occurs, the crack progresses (grows) from the periphery of the substrate to the inside of the substrate as time passes due to stress put on a resin, a film, and the like.